The present invention relates to a brake pressure generator for a hydraulic brake system for use with motor vehicles, comprising a booster piston displaceable in the actuating direction by a hydraulic auxiliary pressure, and comprising a master cylinder including a stepped piston to which pressure can be applied by the booster piston. The piston confines a work chamber and a chamber decreasing in size upon movement of the piston in the actuating direction, which chamber is in communication with the work chamber by way of a check valve and from which a conduit comprising a control valve leads to a non-pressurized container.
A brake pressure generator of this type is described in published German Patent Application P 34 28 136.3. According to the patent application, the brake pressure generator comprises a hydraulic force booster in the pressure chamber of which a hydraulic auxiliary pressure is adjustable by means of a brake valve which auxiliary pressure is proportional to the operating force respectively exerted on the brake pedal. A component part of the hydraulic force booster is a booster piston which, in a sealed manner, is displaceably disposed in a bore of the hydraulic force booster to displace a master cylinder piston in the actuating direction as soon as an adequately high pressure has built up in the pressure chamber of the hydraulic force booster. The booster piston is substantially of a stepped-piston configuration, with an annular chamber being formed between the piston step of smaller diameter and the housing of the brake pressure generator, which annular chamber decreases in size when the brakes are applied. In the brake releasing condition, the annular chamber is in communication with a non-pressurized container by way of a control valve. When force is applied to the brake pedal, a hydraulic pressure is built up in the pressure chamber of the hydraulic force booster which results in that the valve passage of the control switch between the annular chamber of the hydraulic force booster and the non-pressurized container is closed. As a pressure build-up in the booster chamber of the hydraulic force booster, conversely, results in that the work chambers of the master cylinder are separated from the non-pressurized intake reservoir as a consequence of the displacement of the booster piston in the actuating direction and of the closure caused thereby of the central valves provided in the master cylinder pistons. With a continued displacement of the booster piston, the pressure fluid volume escaping form the annular chamber, by way of a control valve, is directly supplied to the one of the work chambers of the master cylinder thereby initiating a relative movement between the master cylinder piston and the booster piston, with the master cylinder piston moving faster than the booster piston to allow high master cylinder pressures to be generated by relatively low pedal paths. In case of a failure of the auxiliary pressure supply, the control valve is held in its open position as no pressure is built up in the pressure chamber of the hydraulic force booster. By applying the brake, the booster piston is then displaced alone by exerting force to the brake pedal, with the volume displaced from the annular chamber escaping into the non-pressurized container. Hence, no transmission will take place by displacement of the volume into the work chamber. The prior art brake pressure generator is largely satisfactory in operation in that the desired abrupt change in ratio is attained in a favorable manner. However, it is not suitable for use with brake systems including a slip control.
It is, therefore, the object of the present invention to provide a brake pressure generator of the type described that can be used with brake systems comprising a slip control.